<jats:title>Abstract</jats:title><jats:p>The antioxidant activities of several extracts from Susabinori <jats:italic>(Porphyra yezoensis)</jats:italic> were measured by the ferric thiocyanate method and the thiobarbituric acid method. The methanol, acetone, ethyl acetate, and hexane extracts, and the chloroform‐soluble and water‐soluble fractions from the chloroform‐methanol extract exhibited higher activities than α‐tocopherol. The hot water extract showed little activity. Thin‐layer chromatography analysis of the active extracts suggested the existence of several antioxidants. The activity of the chloroform soluble fraction was due to chlorophyll analogs. A strong antioxidant was isolated from the methanol extract, accompanied by several amino acids such as leucine and phenylalanine. This compound was identified as usujilene, a kind of mycosporine‐glycine like amino acid.